The nature of the host-tumor (parasite) relationship in the Shope-Fibroma of the rabbit is being studied. Innoculation of Shope-Fibroma Virus (SFV) into adult rabbits results in a self-limiting skin tumor which is rejected after 10-13 days by T-cells. Innoculation of neonatal rabbits produces a systemic infection of the reticulo-endothelial system with metastatic fibromas and death of some animals. In order to determine the difference in response of the neonatal rabbits and of the adult the development of antibody and cell mediated cytotoxicity to SFV in neonatal rabbits will be assayed. The effects of adoptive transfer of lymphoid cells of different types from regressor adults to neonates will be determined and the effects of neonatal cells on adult immune responses in vitro tested. In addition, attempt will be made to establish fibroma cells in vitro and in nude mice. These studies should clarify host tumor immunity in this virus induced tumor system.